There are known some forms of surface acoustic wave convolver, i.e. (a) medium-spaces convolvers where a semiconductor and a piezoelectric material are united via a slight space therebetween, (b) elastic convolvers which use the non-linearity of a piezoelectric substrate itself, and (c) monolithic convolvers where a semiconductor substrate and a piezoelectric film deposited thereon form a laminated structure.
Medium-spaced convolvers at (a) above are inferior to others in productivity because they require a careful control of the space. Elastic convolvers at (b) above require a large surface acoustic wave energy because they utilize the elastic non-linearity of the piezoelectric substrate. However, elastic convolvers as well as laminated convolvers at (c) above, unlike medium-spaced convolvers, can be readily assembled and are good in productivity. Laminated convolvers are also excellent in convolution efficiency because they utilize the nonlinearity of depletion layer capacitance in a semiconductor, and are simple in arrangement due to the monolithic structure.
FIGS. 14 and 15 are a cross-sectional view and a plan view of such a laminated structure. Reference numeral 1 designates a silicon or other semiconductor layer, 2 denotes a piezoelectric layer made from zinc oxide, aluminum nitride or other material, 3 refers to a surface acoustic wave input transducer, and 4 shows an output gate. The transducer 3 and the gate 4 are aluminum or other metal films.
It is widely known to use a surface acoustic wave convolver as a signal processing element in spread spectrum communication systems, for example. In spread spectrum communication systems, DPSK is often used for data modulation.
DPSK-modulated signals can be matched and filtered by a surface acoustic wave convolver having the construction of FIG. 16, for example. The structure of FIG. 16 is called "DPSK convolver" detailed operations of which are disclosed by D. Brodtkorb and J. E. Laynor in pages 561 through 566 of Ultrasonics Symposium Proceedings, 1978.
In FIG. 16, reference numerals 5 and 6 are spaced output gates. 7 denotes a hybrid for summing and subtracting signals from both gates 5 and 6. 8 and 9 designate hybrid outputs for outputting a sum (.SIGMA..sub.out) from 8 and a difference (.DELTA..sub.out) from 9.
Still referring to FIG. 16, when a surface acoustic wave travelling from the left-end input transducer 3 to the right, for example, reaches the right-end input transducer, it is reflected by the right-end input transducer into a surface acoustic wave travelling to the left. The reflected, leftward travelling surface acoustic wave reacts on a rightward travelling wave from the left-end input transducer to produce a convolution output. This output generates undesired waves which are called self convolution. This phenomenum is caused when the right and left input transducers are regular transducers, but may be removed by using a uni-directional transducer. However, unidirectional transducers are usually difficult in design and manufacturing. Additionally, since spread spectrum communication systems generally deal with wide band signals, unidirectional transducers must have a wide band characteristic, and this makes their design and manufacturing more difficult.
Japanese Patent Publication No. 47569/1972 discloses a surface acoustic wave device adapted to decrease undesired waves. However, the publication does not teach the use of the device as a DPSK convolver.
A spread spectrum communication receiver using the DPSK convolver of FIG. 16 is significantly expensive because it uses the hybrid 7.